RC Car Robot

There is a chip on the small board which is edge on in the photo’s - That’s the control chip!.

The leads from the 4 chunky things on the right of the photos should go to the main drive motor if so they form the h-bridge.

The 2 small black things with 3 legs just to the left of what I think is the h-bridge are the control transistors for the main drive h-bridge.

The group of 6 small 3 legged black things on the lower left should have leads running to the steering motor. I think they form the h-bridge for the steering motor along with its control transistors.

The rest is the RF stage plus a few bits and bobs.

You need to get the id number of the control chip.

Mark

PS Tanks are much easier to convert than cars!

M

Thanks for the info! The text on the control chip is incredibly tiny, and reading it reminded me that I'm due for an eye doctor appointment.

Anyway, the ID appears to be SDRX2BDS, and there is a string of numbers beneath it that reads 11120006. I've not yet Googled, but am about to do so.

Ok, a little more information: I don’t have access to the back of the chip itself; all I see is whatever material the board is made of. Instead, pertinent inputs and outputs to the chip appear to be broken out (bottom-left of the attached photo). Looking out F, B, V+, and G, I think I can trace back to the pins on the chip that I need. I have a meter I can use to check current and such across all of them in various motor states. Question: can I solder directly to the broken out connections in the bottom-left, or should I make my connections elsewhere?

Here are some more stupid questions: what kind of wire do I need to purchase? Are there any special bits of hardware that I can put on the ends of the wires to enable me to connect them to the Arduino without having to solder anything to it?

I apologize for the pathetically noob-ish questions. Just treat me like a slightly advanced two-year-old and we’ll be fine. :slight_smile:

TheWumpus:
Thanks for the info! The text on the control chip is incredibly tiny, and reading it reminded me that I’m due for an eye doctor appointment.

Anyway, the ID appears to be SDRX2BDS, and there is a string of numbers beneath it that reads 11120006. I’ve not yet Googled, but am about to do so.

You don’t have to - that’s the RX2 half of the TX2/RX2 chipset (if you look inside the controller, you’ll probably find something labeled “SDTX2BDS” or similar). Now, go to that thread I mentioned, and read it. There’s a link in that thread to the PDF of the spec sheet for the chip.

You’re first issue is to determine what the power source is for the chip, and what it is outputing as a logic HIGH on it’s control outputs. You’ll need a multi-meter for that. Once you know both of those things, then you’ll be set to go to the next step.

I see that the chip is on a “daughterboard” of a sort. That’s both a good thing and a bad thing. Good, because it means you can trace with your multimeter and your eyes the leads from the chip to the pins/holes on the “main” board. Bad, because it makes checking the pins and such a bit awkward (especially those near the main board side). But once you have things traced out, you can then carefully remove the daughterboard, and solder wires in place where the pins went, then supply signals to those pins somewhat directly from the Arduino. You may need to add current limiting resistors in between (if they are on the daughterboard and not the main board; hopefully they are on the main board side). Likely power and ground for the daughterboard come from the main board, so those pins may make things really convenient.

But you first need to figure out those voltages. Likely it will be either 3 volts or 5 volts used for the power to the chip and logic HIGH; if the chip is running on 3 volts, then the logic high will likely be 3 volts or close to it. One way to make a “first guess” is to find out what kind of battery it uses; if it is a 6 volt or greater battery, likely it is running the chip at 5 volts (which makes things much easier to interface with the Arduino). Otherwise it is probably a 3 volt system, which means you’ll want to do some level shifting on the outputs (you can do this with resistors or diodes, or you can purchase specialty 3.3V <=> 5V level shifters; Sparkfun, among other vendors, sells them).

If you keep the receiver daughterboard intact, and you label the pins on the edge in some manner - you can use that for other control projects (with the transmitter)…

Just take your time, and be very patient and careful, documenting everything along the way. Read that thread I mentioned; you’ll note that the OP of that thread ended up messing up his first car, but then got the second working (though IIRC, that one died too) - so be aware of that possibility. This isn’t a “fool proof” operation, but many people have done it.

Good luck!

TheWumpus: Ok, a little more information: I don't have access to the back of the chip itself; all I see is whatever material the board is made of. Instead, pertinent inputs and outputs to the chip appear to be broken out (bottom-left of the attached photo). Looking out F, B, V+, and G, I think I can trace back to the pins on the chip that I need. I have a meter I can use to check current and such across all of them in various motor states. Question: can I solder directly to the broken out connections in the bottom-left, or should I make my connections elsewhere?

As I noted before - once you know what pins do what (and the voltages needed) - REMOVE the daughterboard, then make the connections to those pins (hey, its a convenient breakout, right there). In fact, I would look into getting a breakaway male or female header (like those used on the Arduino, for instance - those are female headers), and solder it in place - then you can easily breadboard and play...!

TheWumpus: Here are some more stupid questions: what kind of wire do I need to purchase? Are there any special bits of hardware that I can put on the ends of the wires to enable me to connect them to the Arduino without having to solder anything to it?

I would use some ribbon cable personally (6 inches or so) - put a header to match whatever you put on the board (if you put a female header on the board, put a male header on your ribbon cable, on both ends). Solder it in place, then put some hot glue or such over the connections to make stabilize and insulate it.

Hope that helps.

:)

I was looking at the pictures again - and noticed something - I think all the base resistors for the on-board transistors (for the h-bridges) are all on the main board. For instance, see resistors R2, R3, R4, R5 - they all seem to have the same value (though I can’t tell the colors easily, so I’m not positive - but I think 2K ohm?). It seems R3 is driving the base of Q2, and R2 the base of Q1, and R5 the base of Q? (can’t see it - maybe Q6?).

Anyhow - I’d be willing to bet that the transistors (or mosfets?) on the right side of your first pic (the large squarish things next to the GS-0020R marking) are the driver transistors for the drive motor. Likely they are being triggered in pairs by Q1 and Q2 (via base resistors R2 and R3 respectively). The other 6 transistors are likely used to drive the steering actuator (likely a simple “bang-bang” full left-right electromagnetic actuator). Two of them will again be for the opposite driver “legs” of the h-bridge (likely Q? and Q9 in the middle, but I don’t know for certain).

Whatever you do, -don’t- turn on both transistors at the same time, or you’ll have a shoot-thru issue which will fry the h-bridges.

Hope this gives a bit more insight. I’ve always found it fascinating how this chipset has been implemented “in the wild”…

BTW, TheWumpus -

I've been meaning to (for a long time - since that mega thread) to write up an article (for my website, maybe as a PDF or such - not for sale, all free) on all of this; would you mind if I used your photos of your board, plus any other insights you post here - as part of that project? I'm just wanting to make it something more clear and such. I would give you full attribution on the images and anything else, of course...

Wow, thanks again. This is a huge help. I'll read the other thread thoroughly and report back as I make progress.

Also, feel free to use anything I post, and don't bother with crediting the photos. I only took them to extract useful information from you all! In fact, if you want better/clearer shots, let me know.

Alright, so the battery itself is 7.2 V (this is written on it, and I verified it with the multimeter). I had my wife control the drive motors forwards, and held the positive lead on my multimeter at the point where I believed the chip was outputting high for forward. I put the negative lead of the multimeter on the ground pin on the PCB. When driving forwards, I measured 1.6 V. I then repeated the process for the pin that I believed was reverse, and measured the same. The same was true for left and right turning (though they were closer to 1.7 V), though I believe the labeling on the PCB has right and left reversed for some reason.

So now what? Are these voltages indicative of a 3 V system or a 5 V system? I realize that may be a stupid question, but while the voltages are nearer to 3 V than 5 V, they're not very close to 3, and I wasn't sure if the voltage was being stepped down somehow.

Also, do I need to be checking the current across these as well?

TheWumpus: Alright, so the battery itself is 7.2 V (this is written on it, and I verified it with the multimeter). I had my wife control the drive motors forwards, and held the positive lead on my multimeter at the point where I believed the chip was outputting high for forward. I put the negative lead of the multimeter on the ground pin on the PCB. When driving forwards, I measured 1.6 V. I then repeated the process for the pin that I believed was reverse, and measured the same. The same was true for left and right turning (though they were closer to 1.7 V), though I believe the labeling on the PCB has right and left reversed for some reason.

Regarding the left/right thing - I have seen this kind of thing before with this chipset; that the implementation is in reverse to what the datasheet says. I am not sure why this is - whether it is the manufacturer (or the car), or the datasheet that is at issue. Just as a long as you realize it, it shouldn't be an issue.

TheWumpus: So now what? Are these voltages indicative of a 3 V system or a 5 V system? I realize that may be a stupid question, but while the voltages are nearer to 3 V than 5 V, they're not very close to 3, and I wasn't sure if the voltage was being stepped down somehow.

If you can in some manner, check the voltages without the outputs of the IC connected to the base resistors; likely the rest of the circuit is "getting in the way".

TheWumpus: Also, do I need to be checking the current across these as well?

I'm not sure what that means; if you are under the impression that things "output" current, you are mistaken. Current is "pulled" from a device, not "pushed" to it. That's the best layman's way I can put it. It depends on the resistance of the circuit and the supply of the voltage across that resistance - ohm's law - I=V/R - so if you have -zero- resistance (ie, a short) the current effectively becomes "infinite". If you have a break (an open), thus "infinite" resistance, then the current effectively becomes zero.

Now - if you do "cut the trace" between the output of the RX2 IC and the base resistor, and you can stick you meter in between and measure the current flow; that might be helpful (once you know the voltage output) - to know if by hooking up the circuit you'll pull more current than what the Arduino (or whatever you use) can supply (in that case, you might need to include a buffer circuit or something to help supply more current).

cr0sh: I'm not sure what that means; if you are under the impression that things "output" current, you are mistaken. Current is "pulled" from a device, not "pushed" to it. That's the best layman's way I can put it. It depends on the resistance of the circuit and the supply of the voltage across that resistance - ohm's law - I=V/R - so if you have -zero- resistance (ie, a short) the current effectively becomes "infinite". If you have a break (an open), thus "infinite" resistance, then the current effectively becomes zero.

Now - if you do "cut the trace" between the output of the RX2 IC and the base resistor, and you can stick you meter in between and measure the current flow; that might be helpful (once you know the voltage output) - to know if by hooking up the circuit you'll pull more current than what the Arduino (or whatever you use) can supply (in that case, you might need to include a buffer circuit or something to help supply more current).

My recollection of current is that it is analogous to flow (of, say, water), measured in coloumbs/second, which I know is amperes. Voltage is joules/coloumb, and specifies how much "punch" each unit of charge has.

In any case, I was under the impression that too much current through a circuit would fry something. For example, let's say the RX chip outputs 5V for pushing the car forward. Can I really just connect a wire from an output on the Arduino to the location on the board where the chip was outputting its 5V signal and everything will work? Do I need a resistor in there? If I understand you, the answer is no, and the circuit itself will dictate the current.

cr0sh: If you can in some manner, check the voltages without the outputs of the IC connected to the base resistors; likely the rest of the circuit is "getting in the way".

Any idea how I can do this?

My recollection of current is that it is analogous to flow (of, say, water), measured in coloumbs/second, which I know is amperes. Voltage is joules/coloumb, and specifies how much "punch" each unit of charge has.

This is a good analogy. The next sentence should logically read something like "Therefore, it's obvious that a tiny diameter pipe connected to the water source will have a very high resistance, and therefore the flow into it will be small, while a large diameter pipe will have a low resistance to water flow".

oric_dan(333): This is a good analogy. The next sentence should logically read something like "Therefore, it's obvious that a tiny diameter pipe connected to the water source will have a very high resistance, and therefore the flow into it will be small, while a large diameter pipe will have a low resistance to water flow".

True enough. I take it, then, that resistance is a result of both the wire itself and by things like resistors and the motors. In that case, assuming the output voltage for my Arduino matches that of the IC output, the only concern becomes whether or not the Arduino outputs can allow enough current to pass to meet the requirements of the motor control board, which I believe is something that cr0sh pointed out earlier. Do I have that right(ish)?

I also drew a rough sketch of how I believe the circuit is designed for the motor controller. F_ic and B_ic are forward and back on the IC, respectively, and M_f and M_b are motor forwards and backwards, respectively. I’m sure I violated all sorts of standards for circuit diagrams, so forgive me, but I think the general idea is there.

Scan.jpeg

TheWumpus: In any case, I was under the impression that too much current through a circuit would fry something. For example, let's say the RX chip outputs 5V for pushing the car forward. Can I really just connect a wire from an output on the Arduino to the location on the board where the chip was outputting its 5V signal and everything will work? Do I need a resistor in there? If I understand you, the answer is no, and the circuit itself will dictate the current.

It may work, if the circuit doesn't pull more current than the Arduino can safely supply on a pin (about 25-30 mA tops; 40 mA is max, but you don't want to go there). If it pulls more current than that - poof goes the pin (well, the ouput driver in the Arduino).

Usually, you want a resistor in there somewhere; start with about 1K, then drop if needed (no lower than 220 ohms). You can probably calculate it, but without knowing the specs of the transistor you are driving, it may not be possible.

TheWumpus:

cr0sh: If you can in some manner, check the voltages without the outputs of the IC connected to the base resistors; likely the rest of the circuit is "getting in the way".

Any idea how I can do this?

Well - you need to locate the base resistor, assuming one exists. Trace from the output pin on the RX2 IC to the transistor it drives - hopefully somewhere there is a base resistor. Then you need to open the circuit at this point - by cutting a trace, or some other method. Then measure the output of the pin to ground. You may want to try first (just in case I'm wrong) to open the circuit -after- the base resistor, and measure the voltage from that point to ground. In case going directly to ground (even with a meter in between) draws too much current (and poof goes the output stage of the IC).

TheWumpus: I also drew a rough sketch of how I believe the circuit is designed for the motor controller. F_ic and B_ic are forward and back on the IC, respectively, and M_f and M_b are motor forwards and backwards, respectively. I'm sure I violated all sorts of standards for circuit diagrams, so forgive me, but I think the general idea is there.

In general it looks OK, though you show the output to the motors from the base of the transistors, and it's not really the "standard diagram" of an h-bridge you normally see, but close enough I think. You might want to compare it to the reference diagram in the PDF of the datasheet (I also note not seeing any flyback diodes - which is crucial in an h-bridge design based on transistors - unless those are -not- transistors, and are mosfets instead, which have an in-built diode across the source-drain; this would explain some things)...

I noticed you drew on one pair of transistors the base resistors, but not the ones between the RX2 IC and the first set of transistors - were there no resistors?

Looking at your diagram, the transistors F(ic) and B(ic) are probably being used in a couple of ways: 1) as a means to switch -opposite pairs- of the transistors in the h-bridge on/off (to prevent shoot-thru - which is bad - causes lost smoke), and 2) probably as a way to boost the output current of the RX2 IC to drive the larger transistors in the h-bridge.

If you can figure out or read the part number of these "buffer" transistors, it would be best to know that information. You can then determine (from the datasheet for the transistor) whether you can use it with the Arduino, what kind of base resistor would be needed, etc. Knowing the part number and such for the main drive transistors would also be helpful (especially if they are mosfets and not bipolars, which I am starting to seriously suspect).

You might be able to drive those transistors via their base resistors (I'm also surprised here where two of the other base resistors went?); it would use four digital outputs, of course (you might want to use a buffer driver IC in between), and you would have to code things carefully so that they get switched on/off properly without leaving two of the transistors on the same side of the bridge "on" (shoot-thru). Port manipulation would be best here (with all pins on the same port), or you might be able to carefully craft the software so that, knowing the last state of the pins, it would know how to set the state properly to avoid the shoot-thru condition...